I. Field of the Invention
The present invention relates to a time domain measurement and control system for a hot wire air flow sensor.
II. Description of the Prior Art
Many automotive engine systems utilize hot wire air flow sensors in order to determine the air flow rate into the engine. This air flow rate, in conjunction with other engine parameters, is then utilized by a fuel management system which regulates and controls the operation of the engine.
These previously known hot wire air flow sensors utilize a resistive element based upon a linear analog proportional feedback control signal. This control signal varies the current flow through the resistive element by an amount sufficient to maintain the temperature differential between the resistive element and ambient air at a predetermined constant, typically 200xc2x0 C. The magnitude of the current flow through the resistive element is then proportional to the air flow through the air flow sensor.
These previously known analog control systems for hot wire air flow sensors, however, have not proven wholly satisfactory in operation. These previously known systems suffer from relatively slow response times and relatively high steady state error. Furthermore, the required warming time for the air flow sensor in many cases is unsatisfactory.
A still further disadvantage of these previously known analog control hot wire air flow sensors is that the control system for such air flow sensors required operational amplifiers, a Darlington pair transistor and other resistors and capacitors to provide the correct feedback gains and offsets for the air flow sensor. As such, these previously known control systems for hot wire air flow sensors were relatively expensive due to their multiplicity of components.
The present invention provides a control system for a hot wire air flow sensor which overcomes all of the above-mentioned disadvantages of the previously known devices.
In brief, a preferred embodiment of the control system of the present invention comprises means for generating a fixed frequency but variable width pulse train through the heating element. This pulse train, furthermore, has a first predetermined voltage amplitude at the input end of the heating element for the hot wire sensor. Although any conventional means can be utilized for generating the pulse train, in the preferred embodiment of the invention, a microcontroller, preferably microprocessor based, is programmed to generate the fixed frequency, variable width pulse train at an outlet port on the microcontroller.
Means are then provided for determining the output voltage amplitude of the pulse train at the output end of the heating element for the air flow sensor. In the preferred embodiment of the invention, the pulse train at the output end of the heating element is coupled as an input signal to an analog/digital (A/D) converter on the microcontroller. The digitized output from the A/D converter is coupled as an input signal to the microcontroller which then varies the duty cycle of the pulse train by an amount sufficient to maintain the output voltage amplitude at the outlet end of the heating element at a second predetermined voltage level. By doing so, the temperature differential between the heating element and ambient air is maintained substantially constant, e.g. 200xc2x0 C.
The duty cycle of the pulse train is thus proportional to the energy consumption of the hot wire heating element which, in turn, is proportional to, i.e. varies as a function of, the air flow through the air flow sensor. Consequently, the duty cycle of the pulse train is proportional to, i.e. varies as a function of, the air flow through the air flow sensor and forms an input signal to the engine management system for the vehicle. Furthermore, as used in this application the word proportional means to vary as a function of, but not necessarily linearly.
In a modification of the present invention, a second measurement pulse train having the same frequency but mutually independent of the first pulse train is generated through the heating element of the hot wire air flow sensor. The amplitude of the test pulse train, rather than the main pulse train, i.e. the pulse train which actually heats the resistive heating element, is then used to vary the duty cycle of the main pulse train so that the amplitude of the second pulse train at the outlet end of the resistive heating element is maintained substantially constant.
In a still further embodiment of the present invention, the microcontroller generates a fixed width but variable frequency pulse train through the heating element of the air flow sensor. As before, the amplitude of the pulse train is determined at the output end of the resistive heating element and the frequency of the pulse train is varied in order to maintain the amplitude of the pulse train at the output end of the heating element substantially constant. In this embodiment of the invention, the frequency of the pulse train is proportional to the air flow rate through the air flow sensor.
Any conventional means, such as a PID controller, can be utilized to vary either the pulse width or pulse frequency in order to maintain the amplitude of the pulse train at the output end of the heating element substantially constant.